This invention relates to a closure system for resealably attaching a tape tab fastener to a fabric surface. More particularly, this invention relates to the provision of such a system to a disposable diaper or incontinent device having a fabric covered backing.
Disposable diapers provide substantial advantages in convenience over diapers intended to be laundered and reused. Typical disposable diaper structures comprise a moisture-retaining layer of high liquid-holding capacity and a moisture impervious backing sheet therefor generally made of a plastic film such as polyethylene film or the like. Typical disposable diaper structures are shown in U.S. Pat. No. 3,612,055 and U.S. Pat. No. Re. 26,151.
As may be seen from the above cited patents, it is desirable to obviate the problems that are inherent in closure systems which utilize extraneous fasteners such as safety pins, snaps and zippers. U.S. Pat. No. 4,540,415 discloses a disposable diaper which has a pressure sensitive adhesive closure, a moisture-permeable facing sheet and a polyethylene moisture-impermeable backing sheet. The pressure-sensitive adhesive closure permits reopening the diaper numerous times without substantial damage to the polyethylene backing sheet or to the adhesive closure. However, it has recently been proposed to provide diapers and incontinent devices which have fabric covered backings and the closure of U.S. Pat. No. 4,540,415 is not suitable in connection with fabric covered diapers or incontinent devices when it is desired to reopen and reseal the closure system numerous times while still permitting the tape tab fastener to bond and hold tenaciously to the fabric surface.
It has been a challenge for pressure sensitive adhesive scientists to design an aggressive adhesive for making a strong bond to fabric surfaces. The difficulties with bonding to fabric are associated with the irregular surface to which the adhesive has to bond and still hold tenaciously. Furthermore, the problem becomes much more complex if there is a requirement for the closure system to be repositionable or resealable. The first important criterion is that the fabric should not have loose fibers which are picked up by the adhesive surface, thus making the adhesive non-functional. The loose fibers may be tied down by applying a hot melt coating on the fabric surface or by saturating the fabric with a hot melt composition. In accordance with the invention, it has been discovered that several important parameters must be met in order to prepare a successful coating from said hot melt composition. There criteria are as follows:
1. The coating must be non-tacky. PA0 2. It must have good heat, light and aging stability. PA0 3. It must be odor free. PA0 4. It must possess low viscosity for good penetration and saturation into the fabric. The hot melt coating utilized in accordance with the present invention should have a viscosity of not more than 15,000 cps at 350.degree. F. In this connection, it is desirable that the viscosity be less than 3,000 cps. PA0 5. The pressure sensitive adhesives which are used according to the present invention and which are defined hereinafter, must have high shear values with respect to said coating. The 180.degree. shear of the pressure sensitive adhesive, with respect to the coating, should be at least 15 lbs/inch although 25 lbs/inch is desirable. PA0 6. The 90.degree. peel of the pressure sensitive adhesive defined hereinafter, with respect to said coating should be at least 0.7 lbs/inch, although the preferred range is between 0.8 and 1.2 lbs/inch. PA0 7. The ingredients in the hot melt coating must be non-contaminating with respect to the pressure sensitive adhesive so that repeated reseals can be achieved. In this connection a light ball tack measurement value should not change in excess of 15% when read after the first and after the fifth reseal. PA0 (A) a hot melt polymer; PA0 (B) between 10 and 50 parts by weight per 100 parts by weight of said hot melt polymer of a modifying resin having a number average molecular weight of between 500 and 3,000, the viscosity of the mixture of said hot melt polymer together with said modifying resin, being between 1,000 and 15,000 cps at 350.degree. F.; PA0 (C) between 0 and 40 parts by weight per 100 parts by weight of said hot melt polymer of a thermoplastic rubber of the A-B-A or A-B block copolymer family, with the proviso that when the amount of said thermoplastic rubber is greater than 0, then the amount of modifying resin may constitute up to 100 parts by weight per 100 parts by weight of said hot melt polymer. The 180.degree. shear of the pressure sensitive adhesive, with respect to the landing area, is at least 15 lbs/inch, 25 lbs/inch being preferred. The 90.degree. peel of the pressure sensitive adhesive with respect to the landing area is at least 0.7 lbs/inch, PA0 a range of 0.8 to 1.2 lbs/inch being preferred. The light ball tack measurement value of the pressure sensitive adhesive does not change more than 15% when read after the 1st and 5th reseal of the closure system. In view thereof, the closure system may be reopened and resealed numerous times while still permitting the tab to bond and hold tenaciously to the landing area of the fabric surface.
Although there are numerous low viscosity (less than 15,000 cps at 350.degree. F.) hot melt polymers or formulations available in the marketplace, none of them have been found suitable, per se, for the purpose of the present invention. Plasticizers or modifiers are frequently added to commercial formulations. These additions tend to migrate and become contaminants to the adhesive. Most of the known hot melt polymers are based on polyethylene, ethylene vinyl acetate, polyesters, polyamides and chemically modified polyethylenes or composites therewith. The known hot melt polymers were evaluated together with commercially compounded formulations but none of these polymers met all of the above listed criteria. The greatest failure among the known hot melt polymer formulations occured with respect to the 90.degree. peel and 180.degree. shear. In addition, there was rapid detackification of the pressure sensitive adhesive when comparing the first and the fifth reseal.
It has been discovered, in accordance with the present invention that the 90.degree. peel and 180.degree. shear and the resistance to detackification has been greatly improved and this improvement has made possible a repositionable closure for fabrics. In this connection, it has been found that particularly with respect to infant diapers in the toddler stage, it is desirable to be able to open the diaper to inspect it to see if the diaper needs changing. When the closure system of the present invention is utilized in connection with a diapers, in which the backing sheet is covered with a fabric, the closure is easily removed for inspection, and then just as easily re-adhered to the hot melt coated area of the fabric which may be woven or non-woven.